Syrian security forces have kept heavy weapons in cities in breach of a UN brokered cessation of hostilities, but the government and opposition both have committed truce violations, a top UN official said Tuesday.

Twin blasts targeting security buildings killed more than 20 people in the northwest Syrian city of Idlib on Monday, a monitoring group said. The violence, a day after the arrival of the chief of a United Nations monitoring mission, was sure to put further strain on a UN-backed ceasefire that went into effect on April 12 but has failed to take hold fully.

Damascus and the United Nations agreed on Thursday on the terms for observers to enter the country to monitor a ceasefire, called more than a year after the start of an uprising against President Bashar al-Assad. But many Western powers remain skeptical the mission will have the clout it needs.